This invention relates to sensing means.
Sensing means, such as vibration or movement sensing means, are commonly used in a variety of applications. For example, such sensing means are used to detect seismic or microseismic events, and may be located either above or below ground. Typically, the sensing means will comprise a number of uniaxial sensors, such as geophones, accelerometers or seismometers, which are capable of sensing components of movement or vibration along one axis only. To determine the full three-dimensional vibration characteristics of an event, it is necessary to use at least three such uniaxial sensors, arranged in a non-parallel configuration so that the components of the vibration in three orthogonal directions can be resolved. For a three sensor configuration for example, in the simplest case the uniaxial sensors can be aligned along the three orthogonal directions, i.e. along the x, y and z axes of a Cartesian system. As mentioned earlier, the sensors to not need to be orthogonal, but merely need to be non-parallel to allow for full three dimensional resolution.
This orthogonal sensor configuration suffers from practical drawbacks however. Uniaxial vibration sensors tend to perform differently depending on their orientation, for example as a result of the effect of gravity. In practice therefore, it is necessary to use one type of sensor aligned vertically (z-axis), and two sensors of a mechanically distinct type which are aligned horizontally (x and y axes). The requirement of using two different sensors types is troublesome as the characteristics of the sensors, e.g. the sensitivity, may vary with each type and thus be difficult to match. In addition, there is more expense involved, and it is more problematic to replace the sensors in the event of a failure.
This problem may be overcome using a different configuration, in which all three uniaxial sensors are arranged at the same angle with respect to the horizontal (Gal'perin, 1984) whilst maintaining orthogonality. The Gal'perin configuration therefore allows a single type of sensor to be used for each of the three sensors.
Recently, it has been proposed to use four sensor configurations. These have several advantages over the three sensor systems, perhaps the most important being the redundancy built into the system, i.e. should one sensor fail, then full three dimensional resolution can still be obtained from the remaining three operative sensors.
A four sensor Gal'perin configuration has been proposed, which comprises the normal three sensor Gal'perin configuration, with an extra vertically arranged sensor (Gal'perin 1984 and Morozov et al 1997). Here, the angle between the sensors is not equi-angular, some of the pairs of sensors are orthogonal and some are not. This configuration obviously suffers from the drawback that a sensor of a different type must be used for the fourth, vertical sensor.
An alternative four sensor configuration that has been proposed is known as a tetrahedral configuration. In this case, the sensors are arranged along the four vectors that join the centre of a regular tetrahedron with each of its four vertices. In this configuration, all of the sensors are equi-angular, having an angle of 109.471.degree. between the axes of any two sensors, while there is no orthogonality. Such an arrangement is described for example in WO 02/068996. This configuration has the advantage that the sensors can be easily checked, as the summed output of the sensors should equal zero if all sensors are functioning correctly. This property arises as a result of the tetrahedral geometry of the sensors. Again however, this arrangement has the disadvantage that different types of sensors may have to be used, depending on each sensor's angle to the vertical.
It is an object of the present invention to provide a four sensor configuration which has the advantages of a tetrahedral configuration, while removing the need for different types of sensor.